The present invention is generally directed to printed circuit board processes and products. More particularly, the invention relates to the formation of fine pitch solder deposits on printed circuit boards.
Surface mount technology is now routinely used to attach packaged integrated circuit devices to printed circuit boards. In general, the practice involves the application of solder paste to copper contacts of the printed circuit board through a stencil patterned with openings corresponding to the board contact locations. The solder paste is screen deposited in patterns on the printed circuit boards using the stencil as a mask and a doctor blade to squeegee the solder paste through the holes in the stencil. When the stencil is removed, the solder paste remains on the printed circuit board contacts.
Since the solder paste is typically 50% flux by volume, with the other: 50% being particles of solder, the paste is also used to hold the component terminals in place during the solder reflow step which follows. The eutectic low melting point solder (63% tin, 37% lead-63/37) normally used permits reflow and a concurrent bonding of the component terminals to the printed circuit board contact at a temperature (below 250 deg. C.) compatible with the glass transition capabilities of flame retardant level 4 (FR4) printed circuit board materials.
The spacing of the leads for packaged integrated circuit components typically exhibit a pitch no finer than 16 mils. This capability is consistent with conventional surface mount technology (SMT) processes using screen deposited solder paste to hold and connect the packaged integrated circuit devices to printed circuit boards.
An integrated circuit connection technology using finer, less than 16 mil, pitch has been developed and is being used with advanced computer systems. The technology is generally known as flip-chip attachment, otherwise technically identified as controlled collapse chip connection (C4). Instead of attaching the integrated circuit die to a lead frame in a package having the leads or pins for mounting, the C4 design involves the formation off an array of solder balls on the surface of the integrated circuit die itself. The solder balls are composed of high melting point solder (3% tin, 97% lead-3/97) at an approximate pitch of 10 mils. The C4 die are designed to be connected to a ceramic substrate, in contrast to a printed circuit board, involving a controlled collapse solder reflow process accomplished at approximately 350 deg. C. This temperature is suitable for the silicon die and ceramic substrate, but not for a printed circuit board.
The high connection count and density of the flip-chip device design makes it particularly attractive for advanced printed circuit board products with numerous integrated circuit chips and extended functionality. This candidacy is reinforced by the fact that photolithographic processes used to form conductive contact patterns on modern printed circuit boards have the capability to create correspondingly fine pitch patterns. Unfortunately, attempts to screen solder paste in the fine pitch characterizing the flip-chip device has proven unsuccessful, in that some of the solder paste lifts off with the stencil, resulting in a non-uniform deposit. These phenomena are attributed to the stencil opening aspect ratio (diameter to thickness), solder paste viscosity and rhedogy, paste formulation, solder paste particle diameter, and stencil material properties. The diameter of the hole is driven by the fine pitch of the flip-chip device, while the thickness is dictated by the need for a minimum volume of solder to connect the ball on the die to the copper contact of the printed circuit board. Experience indicates that approximately 20-80 cubic mils of solder are needed for a nominal 4 mil wide printed circuit board contact to attain reliable connection between the ball of the flip-chip device and the copper contact.
In the absence of an effective solder paste screening process for depositing solder onto fine pitch printed circuit board contacts, the users of flip-chip device on printed circuit boards have developed two techniques for depositing low melting point solder on fine pitch printed circuit board contacts. The first process uses masking and electroplating to deposit the solder. This process includes the formation of a photolithographically defined mask, an electroplate bath deposition of low melting point solder on printed circuit board contacts not covered by the mask, a removal of the mask, and a reflow of the electroplated solder. The process involves numerous steps and has an associated high cost.
The other approach developed to selectively form low melting point solder on the fine pitch contacts of printed circuit boards involves the injection of molten solder through a dispensing head with a mask corresponding to the copper contact pattern of the printed circuit board. Unfortunately, the molten solder dispensing head is very expensive, requires a distinct mask for each different flip-chip device footprint, and dispenses the solder to the contacts of only one die location at a time.
In the context of this known technology, there remains the need for a process, and product formed thereby, which will deposit a low melting point solder on a fine pitch printed circuit board contact pattern within the framework of conventional screened solder paste deposition processes while providing solder volumes on respective contacts adequate to connect conventional SMT components and flip-chip devices.